Hybrid electric vehicles (HEVs) selectively utilize different prime movers, which typically include an internal combustion engine and one or more electric fraction motors, at different points in a drive cycle. A vehicle having a full hybrid powertrain can utilize torque from either or both of the engine and the traction motor(s) for vehicle propulsion. As a result, a vehicle having a typical full HEV powertrain can be electrically propelled immediately upon vehicle launch and while traveling below a threshold vehicle speed. Above the threshold vehicle speed, the engine is automatically started and engaged with a transmission input member.
By way of contrast, the powertrain of a mild HEV lacks the capability of propelling the HEV by purely electrical means. Nevertheless, a mild HEV powertrain retains key design features of the full HEV powertrain described above. Such design features include the capability of automatically shutting off the engine at idle to conserve fuel, and then automatically restarting the engine when needed. In a typical “driver-requested” engine autostart event, the engine automatically restarts when the driver directly requests output torque, usually by removing pressure applied to a brake pedal and/or a threshold amount of throttle request. However, other engine autostart events may be triggered solely by certain vehicle state changes such as a low battery state of charge. As such restart events occur regardless of a driver's input, they are often referred to as “non driver-requested” autostart events.